The mass of the substance (m) liberated at an electrode during electrolysis is directly
proportional to the quantity of charge (Q) passed through the cell.
i.e m α Q
We know that the charge is related to the current by the equation I = Q
⇒ Q = It
∴ m It
m = Z It
Where is Z is known as the electro chemical equivalent of the substance produced of the
When, I = 1A and t = 1sec, Q = 1C , in such case the equation (9.32) becomes, (9.33)
⇒ m = Z …..(9.34)
Thus, the electrochemical equivalent is defined as the amount of substance deposited or
liberated at the electrode by a charge of 1 coulomb.
Electro chemical equivalent and molar mass:
Consider the following general electrochemical redox reaction
M (aq)+ne M(s) n+ – →
We can infer from the above equation that ‘n’ moles of electrons are required to precipitate
1 mole of Mn+ as M(s).
The quantity of charge required to
precipitate one mole of Mn+ = Charge of ‘n’ moles of electrons
In other words, the mass of substance deposited by one coulomb of charge
Electrochemical equivalent of M = Molarmass of M
Z = Equivalent mass
When the same quantity of charge is passed through the solutions of different electrolytes,
the amount of substances liberated at the respective electrodes are directly proportional to
their electrochemical equivalents.
Let us consider three electrolytic cells connected in series to the same DC electrical source
as shown in the figure 9.8. Each cell is filled with a different electrolytes namely NiSO4
When Q coulomb charge is passed through the electrolytic cells the masses of Nickel,
copper and cobalt deposited at the respective electrodes be mNi mCu and mCo , , respectively.
According to Faraday’s second Law,
m Z , m Z and m Z
m Z , m Z and m Z
Z = m